In this section, we are going to describe the LincoSim application usage by means of an how-to set of subchapters that shows in a step-by-step way the possibilities available within the preconfigured workflow activities. In general, the LincoSim platform allows to reproduce in an automatic way the CFD workflow for a given hull under a given working condition. The CFD workflow for hull hydrodynamics problems is constituted by three main elements:
- Inputs: made of two main parts that are the geometry description of the hull and the fluid dynamics conditions at which the hull has to be tested.
- Computing setup: that defines what kind of physics has to be solved and how. In other words the computing setup is the element of the workflow that define what kind of “experiment” we want to virtualize. In hull hydrodynamics study there are at least three type of tests that can be performed: “captive” or zero DoF, one DoF, two DoF.
- Outputs: made of an arbitrary set of KPI values, tables, plots and diagrams that are necessary to support a ranking of the hull performances and support decision-making designer’s activity.
It’s worthwhile comment a little bit more in deep the three types of computing setup that are available for hull hydrodynamics problems since these are the three main choices that a LincoSim end-user can select during his working activity. The zero DoF computational setup is representative of so-called “captive” experimental condition in which the hull attitude is locked. The one DoF setup is used to represent a so-called “free sink” experimental setup in which the hull is free to change his CoG position only by means of a rigid translation along the vertical axis. The two DoF setup is used to represent a so-called “free sink and trim” experimental setup in which the hull is free to change his attitude by means of rotations around the transversal axis as centered in the CoG and by means of a rigid translation along the vertical axis. All these three setup are provided under calm water condition.
Computational Fluid Dynamics (CFD) techniques requires as a first step the import of CAD design shape (the hull). In LincoSim the geometry model import is managed by means of dedicated section (geometry). In figure 2 the geometry load page is shown.
The user is simply requested to select his input cad file and upload it to the system. For sake of compatibility with the selected computational engine the file format accepted is a stereolithography file formatted as ascii (*.stl file). This file format is a very simple geometry representation made of a set of triangles and normal and is available as export file format form all the CAD design software. The geometry file once uploaded is processed, validated and displayed as 3D interactive object before being available for usage within the LincoSim environment. It is worthwhile underline that only validated geometries are available for usage. The coordinate system used is the absolute one and the positive (advancing) direction of the hull is considered as x-axis positive direction; the hull geometry must be prepared accordingly before uploaded.
Geometry input preparation¶
Computational Fluid Dynamics (CFD) techniques require as a first step the import of CAD design shape (the hull). Unfortunately, CAD design constraint and CFD ones are not the same. More important a CAD design that might looks correct in the CAD software can considered as NOT VALID once imported and used for CFD. Moreover, in our application we are moving from a real CAD file format (that depends on the software you are using) to a very simple triangulated file format named Stereolithography (STL file format). Likely, there are in general problems that range from:
- Differences in tolerance
- Presence of gaps due to misleading adjacent surfaces
- Triple connected edges
- Presence of dirty construction edges
- Presence of more than one normal direction (outward or inward)
Disregarding to what the CAD software is used there are some general rules that you can follow to try to provide at design stage a good CAD file definition easily exportable to CFD applications Avoid self-intersecting surfaces. Avoid small and much skewed patch-like surfaces added to join not adjacent surfaces. Cleanup construction lines once they are not useful. Cleanup construction points and in general eliminate multiple unnecessary points. Align all the normal in a coherent way (all outward is the best). STL is the standard file type used by most additive manufacturing systems (3D printer for instance) STL is a triangulated representation of a 3D CAD model. The triangulation (or poly count) of a surface will cause faceting of the 3D model. The parameters used for outputting a STL will affect how much faceting occurs.
When exporting to STL in your CAD package, you may see parameters for chord height, deviation, angle tolerance, poly count, or something similar. These parameters affect the faceting of the STL. Each CAD software has his tips to grant the best quality for exporting to stl. Use best quality. Most important: CFD works with closed Volumes. Use, if available, a closeness check within your cad software before exporting. Watertight STL is mandatory to work with in CFD applications. If a water-tightness check is available in your cad use it.
Working conditions inputs¶
Once the desired hull geometry is correctly uploaded and validated the user can start a virtual experiment using the desired hull geometry by using the new-simulation tab under the simulation page. In figure 4 the new simulation page is shown.
There are two main set of inputs that the user is request to insert in order to start a new simulation: * Basic info * Physical parameters
Basic info are considered:
- Simulation name: is a free entry and represent the name of the simulation in the user simulation database. The name must be composed with at least four alphanumeric characters and not existing in the simulation database.
- Owner organization: is a drop down menu where the user is free to select under which (if more than one) organization desire to perform the simulation.
- Simulation setup: is a drop down menu so that only the simulation setup designed for a given organization are available for selection.
- Machine: is a drop down menu so that only running HPC infrastructures are available for selection.
- Geometry: is a drop down menu so that only the validated geometries are available for selection.
- Availability: is a drop down menu and two kinds of value are allowed (public, private). Public means opened to all the registered users that have access to the LincoSim platform. Private means private to the organization under which the user is doing the simulation.
Instead physical parameters necessary to run the simulation are:
- Hull mass: is a single scalar value that represent the total mass of the hull expressed in SI units.
- Hull Center of Gravity: is tuple of three values that represents the coordinates of the center of gravity (CoG) of the hull in the absolute reference system.
- Hull velocity: is a single scalar value that represents the velocity magnitude of the hull in SI units.
- Water temperature: is a single scalar value that represents the temperature of the water in SI units.
- Hull inertia: is a tuple of three values that represents the value on the diagonal of the matrix of moments of inertia (technically, second moment of area) of the hull geometry computed for the three main axes (xyz) respect to the CoG coordinates expressed in SI units.
- Water z-pos: is a single scalar value that represents the starting z coordinate of the waterline in the absolute reference system.
- Wave height: is a single scalar value that represents the guessed maximum height of the wave in the absolute reference system.
- Hull trim angle: is a single scalar value that represents the starting trim angle of the hull in the absolute reference system computed for a rotation around the CoG.
Notably none of these values is related to any CFD or HPC knowledge, instead all the inputs parameters are strictly related to well-known hull design parameters that can be computed easily with common CAD design tools and software.
There are two main kinds of output automatically provided by LincoSim:
- Synthetic key parameter index (KPI): are analytical values that are typically available as outcomes to the designer in order to rank the hull performance. In particular, LincoSim autonomously compute these main hydrodynamics quantities of interest: total drag, maximum/minimum pressure value on the hull, maximum wave height on the hull, wetted surface area of the hull (wsa).
- Visual data: are 1D, 1D over time, 2D and 3D datasets that are interactively available to the end user to get also a visual insight on a wide range of quantitative outcomes of the performed analysis. Deeper analysis can be performed looking at: pressure patterns on hull, wave patterns, pressure patterns over selected longitudinal lines on hull, forces acting on hull time history, CoG dynamics time history, wetted surface distribution on hull.
It is worthwhile underline here that the availability of standardized, automatically coherently computed meaningful hydrodynamics values is a strong benefit of LincoSim.
Complete Single Case Analysis¶
This kind of application represent the average case for which the LincoSim application has been designed. Here below the set of steps involved in a single case analysis and the corresponding LincoSim web pages for a specific hull used to validate the CFD workflow as fully shown in the next chapter.
It is worthwhile underline here that the availability of standardized, automatically coherently streamed workflow is a strong benefit of LincoSim.
Complete Multiple Cases Analysis¶
To support common parametric studies we designed quick and effective access to a submission of a set of simulations in which one of the physical input parameters is free to change in a range defined by the user whereas all other inputs parameters remains constant. The typical example of this kind of analysis is the so-called “resistance curve’ analysis in which the designer need to get an understanding of the total drag value of the hull at different velocity conditions keeping all other physical parameter fixed. Another meaningful example can be a set of captive case with different trim angle. LincoSim allows in a very simple and clear way to perform fat submission of this kind of analysis by means of the “range-simulation” tab selection.
It is worthwhile underline here that the availability of standardized, automatically coherently streamed multiple case workflow is a strong benefit of LincoSim.
Support Request Tools¶
With positive completion of the analysis, the user is allowed to access KPI, visualize, and interactively navigate processed data. Nevertheless, in order to support any kind of error (system error, modelling error, solver error, data processing error or during cad import error), a support request tool has been designed and added to the LincoSim application.
Thanks to these two boxes, the end-user experiencing a problem during his normal workflow stream can easily inform CINECA personnel to give support for the specific given problem. An automatic mailing system contact the technical support.
The automatic CFD workflow (i.e. the simulation setup) proposed is made of three main parts: meshing, computing, post-processing as said. Changes to one or more of these main blocks identify a different workflow type. A general single approach valid for every kind of hull under any kind of working condition is not feasible and it would lead to a very monolithic and rigid approach that from our point of view will be highly inefficient. For this reason, we decide to design our application starting from macro settings, reflecting specific general needs, which can be further customizable upon request.
For instance the three main kind of workflow available for each end-user are: the “captive”, the “1DoF” and the “2DoF” as explained. These three workflow are already designed to be able to work efficiently on specific end-user type of hull. Nevertheless, the three main blocks can be, to some extent, also customized and fine tuned to reach some specific need. This is an important feature of the proposed approach since it allows for maximum flexibility. Once the user identify a specific need and desire to request a customization of the workflow he can provide technical details, including benchmarked reference cases, to CINECA personnel and request the desired customized version of an available workflow. CINECA personnel then perform this kind of activity and the new workflow will appear under the list of available workflow to the end-user.
Geometry and Simulation Dashboard¶
Management of geometry and simulations is another of the key point within the LincoSim application. Thanks to the unified and standardized approach designed by the usage of the LincoSim application, users can visualize in a very friendly and clean way the different geometry cad uploaded to the system and the different analysis performed using the application.
Moreover, due to the intrinsic richness of the dataset, the simulation dashboard has some addition feature with respect to the geometry dashboard. In particular, there are several filter and analysis tools available:
- Filter by name: a standard by name filter is available to sort out only desired simulation sets.
- Filter by simulation status: quick tags selection concerning the actual status of the simulation can be easily performed. The available status are: Completed, Running Created, Error, Deleted
- Filter by organization owner: a user that performs simulations for different organizations can easily pick up only the subset of data related to a selected organization that has been declared as owner during the submission process
- Analyze results using comparative graph for a set of selected hull: using this feature it is possible to compare the simulations selected above with respect to two quantities specified here.
- Analyze results using parallel coordinates for a set of selected hull: using this feature it is possible to refine the search on the selected simulations. Refinement can be accomplished using parallel coordinates technique, i.e. selecting intervals of some of all quantities in the corresponding coordinates. Intervals can be selected brushing them through mouse clicking on the corresponding coordinate.